Field of the Invention
This invention is in the field of printing, more specifically in the field of making flexographic printing plates from liquid photopolymers.
Description of the Related Art
Flexographic printing is a printing process which uses a flexible rubber or photopolymer relief plate, prepared with raised areas called relief areas, corresponding to an image to be printed, and surrounded by lower areas called floor areas. The plate is typically secured to a revolving drum which is contacted by an anilox roll system to coat the relief areas with ink and subsequently presses the inked areas against a moving sheet of, e.g., paper or corrugated box board. The flexible printing plates may alternatively be applied to a plate mount, e.g., hand stamps, or, potentially, non-planar surfaces other than drums.
Images for flexographic printing are typically created by using a negative of an image to mask non-image areas on a layer of photosensitive polymer (“photopolymer”). Photopolymers cure by cross-linking under exposure to ultraviolet (UV) light. The plates may be imaged digitally (what is known as computer-to-plate or CTP) or by the analog process of exposing and developing traditional film. Such films (as supplied by Kodak, Fuji, etc.) are processed into negatives via an image setter.
Another method of making negatives uses an inkjet printer to print a UV-blocking ink onto a clear plastic, e.g., polyester, material. The surface of the plastic material must be inkjet-receptive or made to be inkjet-receptive, that is, able to create an acceptably stable and detailed image, i.e., one that adheres to the plastic and dries quickly without migrating. The plastic material may be made inkjet-receptive by coating it with an inkjet-receptive coating, e.g., a micro-porous coating or a coating of inkjet-receptive polymer, or the material may be treated in some other manner such as acid etching, etc. to produce a surface that allows the ink to adhere to the film and dry quickly without migrating. The uncoated plastic material may alternatively be printed with an inkjet ink directly so long as the ink and the plastic are mutually compatible. The printed image must also have an opacity of at least about 3.0 to prevent UV curing of unexposed photopolymers during the amounts of time required to cure the exposed photopolymers.
Liquid photopolymer flexographic plates are made in an exposure unit having a horizontal bottom glass with a source of UV light below it (lower light), and a lid having a flat top glass with a source of UV light above it (upper light). A traditional flexographic plate is made in the following manner: a negative of the desired image, as prepared by one of the above methods, is placed on the bottom glass, the negative is protected by a thin optically clear film known as cover film. Liquid photopolymer is then cast over the cover film to a predetermined thickness. A substrate for supporting the photopolymer is then laminated over the liquid photopolymer. Thus a sandwich of negative, cover film, liquid polymer and substrate is created. Typically, the substrate consists of a backing sheet of photo-transmissive polymer such as polyester, with a “tie-coat” applied to one side to bond the cured photopolymer to the backing sheet. The substrate is placed in such a manner that the tie-coat is in direct contact with the liquid photopolymer. Liquid photopolymers (such as supplied by Chemence and MacDermid) are cast over the image negative (protected by a cover film) and bond to the polyester backing sheet by the tie-coat.
Precise reproduction of the negative image detail onto the photopolymer requires that the negative be placed as close to the photopolymer layer as possible. In the liquid plate-making process the bottom glass surface is grooved in such a way that, when a vacuum is applied to the grooves, air is removed between the cover film and the relief image negative.
Next, the UV light source in the lid (the upper light) is turned on for a prescribed amount of time to cause the photopolymer adjacent to the substrate to cross-link uniformly over the entire plate, forming the floor. The areas to be imaged are then exposed by the lower UV light (from below the bottom glass) that shines through the clear areas of the relief image negative, which causes the photosensitive polymer to cross-link, forming images that bond to the polymer floor. The (liquid) photopolymer that is not exposed to UV light remains in a liquid state to be later reclaimed and reused. Unexposed sheet photopolymer, by contrast, is not reusable and becomes waste.
The plate-making arrangement is then taken out of the exposure unit and the photopolymer material that has not been cured by UV light is removed from it, leaving the areas exposed to light as relief areas. Un-polymerized liquid photopolymers can be reclaimed by draining and wiping the un-polymerized liquid off the substrate into a suitable container. Photopolymer reclaim can be accomplished manually by squeegee, by a reclaim board with rollers or by an air knife and augmented or enhanced by using heat.
The amount of photopolymer necessary to create a stable floor underneath the relief image portions is reduced in the I-Plate (in-position plate, island plate) process, which adds another step to the above process. Instead of making a floor that extends over the entire plate, a second photographic negative is placed on top of the photopolymer layer. This negative, called a masking film, is a negative that outlines the image areas on the negative with a border of one-quarter to one half inch, more or less. It is predominately black. The relief image negative and masking film are aligned so that each image area of the relief image negative is approximately in the middle of each clear area of the masking film. This prevents a floor area from being created where the masking film is black.
The most commonly-made flexographic printing plates and hand stamps are now manufactured as island plates in the following manner: the first exposure to upper UV light from the lid is through the masking negative, causing islands of cured polymer to be formed beginning in the photosensitive layer adjacent to the substrate. The timing and intensity of the exposure are limited to prevent the polymerization extending all the way through the photopolymer layer from the substrate to the free surface of the layer. The second lower UV exposure, from below the relief image negative, causes the cured detailed relief image to form on top of the islands.
Because of the placement of the second negative above the photopolymer substrate layer, the application of vacuum to remove air from between the negative and the photopolymer substrate layer is less effective than when just the substrate is used. Thus, in this prior art method for producing an I-plate, the increase in recovery of photopolymer comes at the expense of some loss of precision of the relief image. Alternatively the mask negative can be placed above the exposure glass but in this fashion distorts the resulting island formation. The “spread” in the light transmission through the thick upper glass yields an island that is larger than the negative with a broader shoulder. It is also more difficult to register the second negative to the first negative in setting up the process. This problem is solved by the substrate and method for making island plates using liquid photopolymers described in U.S. Application Publication No. 2012/0082932, which is incorporated by reference into this application in its entirety. It employs an inkjet-receptive coating applied to the surface of the backing sheet opposite to the tie-coat. This laminate of tie-coat, backing sheet, and inkjet-receptive coating, known as I-Strate™ and marketed by Chemence, Inc. of Alpharetta, Ga., U.S.A., is a substrate that enables a masking image to be applied opposite to the tie-coat using an inkjet printer. In accordance with the embodiments of the parent application, one single sheet performs the dual role of plastic backing and I-plate masking film.
Parent U.S. application Ser. No. 13/618,507 describes additional features called “I-Strate Plus™.” It differs from the above-described I-Strate™ laminate and method in that the inkjet-receptive coating is not applied directly to the backing; rather adhesive is pre-applied to the backing and covered with a clear, flexible cover film/release liner, as in the Kwikmount™ system, and the inkjet-receptive coating is applied to the cover film/release liner, transforming the cover film/release liner into a surface upon which a negative can be printed. In another embodiment, the cover film/release liner itself is inkjet-receptive, and the negative is printed directly on the liner. This masking negative is removed along with the flexible cover film/release liner immediately prior to mounting the plate. Like Kwikmount™, I-Strate Plus™ can be mounted on a printing plate or roll without the step of applying adhesive to the flexographic plate manually.
An important point to be made about the related art described above is that the inkjet inks used have necessarily been water-based because solvent-based (non-aqueous) inks have heretofore not been sufficiently optically dense. As a result, the above-described print surfaces employ a microporous coating to produce a surface that allows the ink to adhere to the film and dry quickly without migrating. U.S. application Ser. No. 13/902,301, incorporated by reference into this application in its entirety, describes and claims solvent-based inkjet inks and combinations of these with films they are printed on that are result in images that are sufficiently optically dense to create negatives that can be used to expose photopolymers in flexographic plate making.